The present invention relates to a process for sputter coating workpieces in which two sputtering surfaces arranged opposite to each other are mutually sputtered and create between the sputtering surfaces a gas flow that is directed against at least one workpiece. Further, the present invention relates to a sputtering source with two opposite sputtering surfaces, an anode arrangement provided between these sputtering surfaces and a gas exit arrangement effective in a gap defined between the sputtering surfaces, in accordance with the generic terms of claims 1 and 15. Further, the present invention relates to a sputter coating system with a source of said type, as well as the utilization of the process according to this invention, the source according to this invention, and the system according to this invention.
From U.S. Pat. No. 4,094,764 it is known that for sputtering coating a workpiece a gas flow is created across the sputtering surfaces in order to drive the flowing inert or noble gas containing sputtered target material toward the workpiece. In comparison with conventional, cathodic sputtering a much higher coating rate is said to be achievable.
A similar approach is taken according to DE-PS 42 10 125 in which a gas flow is created not across the sputtering surfaces of a flat target according to U.S. Pat. No. 4,094,764 but along the inside surface of a cone-shaped hollow targetxe2x80x94a hollow-cathode targetxe2x80x94and where the material sputtered off the target is directed toward the substrate workpiece by the flow.
In H. Koch et al. xe2x80x9cHollow cathode discharge sputtering device for uniform large area thin film depositionxe2x80x9d, J. Vac. Sci. Technol. A9(4), July/August 1991, the advantages of such hollow cathode sputtering are discussed, in particular with respect to the exploitation of the electrons for ionizing the gas contained within the reaction volume. The proposal in this article is to provide a transverse slot in a target block and to direct a gas to flow through this slot toward the workpiece.
At the slot,two sputtering surfaces of the target are closely opposite each other which minimizes the unwanted deposition on system components other than the targets.
This is particularly significant in conjunction with reactive DC sputtering in which electrically isolating reaction products are created and re-deposited.
However, this approach has the following disadvantages:
To minimize the coating of opposite sputtering surfaces by reacted target material and to maximize the electron impact efficiency for the ionization, the selected gap width and consequently the distance between the two sputtering surfaces must be kept small. However, the design of the gap with closely opposite target surfaces requires correspondingly short gap width surfaces or termination surfaces. This in turn is highly disadvantageous with respect to the electron impact yield in that the electrons are reflected in the end zones of the slot and are ultimately are dissipated via the anode, partially before they have fully released their energy through momentum transfer to the gas particles. When a hollow-slot cathode of the type described in said journal article is used, the basic problem is that the better the target coating problem is solved along the sputtering surfaces, that is, the narrower the slot the greater the loss in electron impact yield in the end zones of the slot. As a rule there is an electron drift in the longitudinal direction of the slot, preferably toward the end of the slot, which leads to uneven longitudinal plasma distribution and consequently uneven sputter distribution.
The objective of the present invention is to solve the aforementioned problems. Based on processes of the aforementioned type this is achieved by making each of the mutually opposite sputtering surfaces closed in themselves, and by creating a gas flow through one of the self-enclosed gaps essentially transversely to the sectional planes in which the gap appears as closed in itself, thus resulting in a closed plasma loop.
As the gap is closed in itself, implemented by two mutually opposite sputtering surfaces that are closed in themselves, for example an inside cylinder target and an outside cylinder hollow target, the result is that the gap width can be designed within broad limits and in particular be short in order to avoid the aforementioned re-deposition problems. But due to the sputtering surfaces that are closed in themselves, even if they feature corners such as when cuboid targets are used, no gap ends exist on the closed loop gap which means that the electrons that move along and within the plasma loop can recirculate therein until they have transferred most of their energy through impacts to the gas particles which leads to a more efficient plasma discharge. This is evidenced by the lower discharge voltages and the more stable operation.
If, as preferably proposed, each self-enclosed sputtering surface is formed on a separate target, this also results in a preferred stable arrangement. To achieve optimum power balance or for producing mixed materials from different target materials, each of these targets can also be fed by a separate generator arrangement.
Due to the high plasma density in the closed loop gap it is possible to further minimize the re-deposition problem on sputtering surfaces, in particular with electrically isolating reaction products, which makes the proposed solution highly suitable for coating with dielectric materials and in particular oxides. In a preferred design at least a portion of the gas is, therefore, chosen as a reactive gas, preferably O2, where good gas separation is ensured if the inert gas, e.g. Ar, is admitted into the gap and the reactive gas, away from the target, is admitted into the chamber.
Reactive gas is admitted with great preference not through the gap but directly into the coating chamber.
In a particularly preferred design, coating is performed with a material containing at least one ferromagnetic component, or with a dielectric material, where both coating materials are known to be highly critical for the DC sputtering process preferred here. In another preferred design deposition takes place with MgO or ITO, be it through reactive sputtering of metallic targets or possibly through additional reactive sputtering of oxidic targets, that is, in particular as in the case of MgO, materials which on account of their low conductance are highly problematic for DC sputtering.
With the procedure according to the invention and based on the fact that the two mutually opposite sputtering surfaces can be implemented on one target each, it becomes possible to use different target materials for the two sputtering surfaces which means that more complex coating materials can be deposited, in particular through reactive sputtering.
In a particularly preferred design a magnetic field is created inside the gap, preferably in the form of a magnetron field, by creating at least one magnetic field with a tunnel-shaped field pattern on at least one sputtering surface, preferably along both sputtering surfaces, where the axes of the tunnel run along the self-enclosed sputtering surfaces and preferably are also closed in themselves.
In comparison with slotted targets described in the aforementioned journal article, a significant increase in the plasma density is achieved by means of the self-enclosed, closed loop gap according to the invention, and the plasma density is further increased by means of said magnetic fields. One of the reasons for this is the increasingly improving electron impact yield (continuously closed loop electron traps).
Although it would be possible to operate the target(s) according to the invention with AC, the distinctly preferred implementation version is to use DC, particularly for cost reasons, possibly as in pulsed mode, with superposed AC in order to achieve particularly high operation stability.
A sputter coating source that solves the aforementioned task is characteristically designed in such a way that each of the sputtering surfaces form surfaces that are closed in themselves, where the gap forms a self-enclosed gap that is open on at least one side, and the gas exit arrangement of the gap opening is located on the opposite side.
A sputter coating system according to the invention features at least one sputtering source of said type as well as a DC generator for the configured targets of which there is at least one, preferably two, and possibly a generator arrangement for outputting a DC signal with superposed AC signal but preferably a pulsed DC signal.
The process according to the invention, the source according to the invention, as well as the system according to the invention are particularly suitable for sputter coating with ferromagnetic materials or dielectric materials, in particular for depositing MgO or ITO coatings, or for coating plasma display panels where large surfaces have to be coated economically.
In this case the substrates can be transported e.g. across the linear source arrangement. It is also possible to arrange several linear sources in intervals suitable for distribution in such a way that the overall coating rate can be increased or large surfaces can be statically coated.
The preferred design versions of the process according to the invention are specified in claims 2 to 14, the source according to the invention is specified in claims 16 to 23, and the system according to the invention is specified in claims 24 and 25.
Particularly preferred applications of the invention are specified in claims 26 to 28.